The present invention relates to count discriminating fire detectors of the type which produces an alarm signal in response to the counting of a predetermined number of pulses generated from the output of a fire sensor, and more particularly the invention relates to such detector which includes a monostable multivibrator so that the production of a false alarm signal due to any undesirable operation of the counter circuit upon connection or reconnection of the power source is prevented.
In the past, a so-called pulse-drive method designed to periodically monitor for fire with a view to reducing the detector current consumption has been used with known photo-electric smoke detectors, ionization smoke detectors and semiconductor heat detectors other than mechanical-contact type fire detectors employing a heat-sensitive member such as a bimetal or diaphragm. In addition, a so-called storage type fire detector has been put in practical use in which to prevent the production of a false alarm signal due to a non-fire cause such as an external noise or tobacco smoke which lasts only a short period of time, an alarm signal is produced only upon continuation of a fire condition over 20 seconds, for example. Since this type of storage type fire detector uses a pulse-drive method as mentioned previously, the function of the detector as the storage type will be made inoperative if the pulse spacing is greater than the storage interval. As a result, where the storage interval is for example greater than 20 seconds, the monitoring for fire is accomplished with an 8-second period pulse spacing so that an alarm signal is produced only when the presence of a fire is determined consecutively over four times, and a digital counter circuit or analog counter circuit is used for counting the number of times the presence of fire is determined.
In the like manner as the ordinary fire detector, a desired number of such storage type fire detectors are connected in parallel between a pair of power supply and signal lines from an alarm receiving panel so that the detectors are supplied with a DC power from the receiving panel through the lines and upon occurrence of a fire the detector sends to the receiving panel through the same lines an alarm signal such as a switching signal which for example establishes a low impedance between the lines. The current consumption of such pulse-operated fire detectors is such that although the pulse width is as short as 100 to 200 .mu.sec, the current consumption per unit may sometime amount momentarily to as much as several hundreds mA. Particularly, since the current consumption of a photoelectric type detector is so large and since a plurality of such pulse-operated fire detectors are connected to the same lines, a large current is drawn from the receiving panel and thus there is the danger of a signal detecting relay in the receiving panel being cause to respond to the large current erroneously and produce a false alarm. As a result, generally this type of fire detector incorporates a large-capacity capacitor as an internal power supply so that the DC power input from the lines is stored in the capacitor and then the required pulse-drive current is derived from the capacitor. Thus a switching circuit for producing an alarm signal is directly supplied from the lines so as to be not influenced by the capacitor. When the power source is connected, due to the presence of the capacitor, the gradually rising power supply voltage is applied to pulse-operated circuit and a counter circuit and the voltage rise time constant is relatively large. To forcibly reset the counter circuit upon connection of the power source is important for the prevention of any false alarm signal upon closing the power supply circuit, and a storage type detector having such reset means is disclosed for example in U.S. Pat. No. 3,842,409 and 4,151,522, in which the reset means comprises a differentiation circuit consisting of a capacitor and a resistor. The problem with this method of forcibly resetting the counter circuit through the differentiation circuit is that although the method is surely effective in preventing any malfunction during the charging period of the large-capacity capacitor upon closing the power circuit, the method has no malfunction preventing effect when upon releasing the supply voltage is applied again to the detector which has produced an alarm signal. More specifically, when the detector produces an alarm signal, its switching circuit establishes a low impedance short-circuit between the lines so that the charging of the large-capacity capacitor is stopped and the capacitor starts discharging its stored charge. Since this discharge takes place through the C-MOS device in the counter circuit, the discharge is effected very slowly and usually the time required for completing the discharge is over 10 minutes. When the alarm signal reaches the receiving panel, for the purpose of confirmation the operated detector is restored to the normal state so as to confirm if the detector again produces an alarm signal, and this releasing operation is usually performed in a time interval shorter than 10 minutes. Consequently, chances are great that reapplication of the power supply voltage upon restoration of the detector takes place before the completion of the discharge of the large-capacity capacitor, with the result that the change in the supply voltage applied to the differentiation circuit is reduced and the counter circuit is no longer reset, thus making it impossible to prevent erroneous triggering of the switching circuit.
This releasing operation will be performed not only in the actually installed fire alarm system but also in the course of adjusting tests of storage type fire detectors prior to their shipping from the factory. Thus the problem of inability to reset the counter circuit has a detrimental effect on the adjusting test works. Usually, for this type of detector the adjustment of sensitivity and storage interval is carried out by repeatedly operating the individual units separately under a simulated fire condition for testing purposes. For instance, in the case of a smoke detector, after the detector has been warmed up sufficiently, the detector is placed in a mass of smoke having a predetermined density to test the detector as to whether an alarm signal is produced in response to the smoke of this density and the time elapsed between the time of placing the detector in the smoke and the time of producing an alarm signal while releasing the detector as occasions demand, and the test process is performed repeatedly. The time required for the detector to produce an alarm after placing it in the smoke is more than the storage interval of over 20 seconds as mentioned previously and whether an alarm signal is produced within the upper limit time of the test criterion such as 60 seconds is confirmed. As a result, the minimum time of 20 seconds is required to make one test on each detector and an additional time of 20 seconds will be required for each releasing of the detector. It will thus be seen that the product testing of the storage type detectors requires a long period of time, that if the sensitivity adjustment is carried out along with the testing, the efficiency of these works will be extremely deteriorated due to the problem of inability to reset the counter circuit and that the efficiency will be deteriorated further if any malfunction takes place upon releasing.